It is known that corrosion of steel in concrete can be reduced or halted by generating movement of ions within the concrete structure between an anode and a cathode defined by the conventional metal reinforcing members within the concrete. Techniques are available for cathodic protection in which sufficient current is generated to maintain an ongoing protection and for restoration in which the current is used for a relatively short time but at a sufficient value to cause restorative effects.
Various restorative effects can be obtained including particularly the extraction of chloride ions from the concrete which would otherwise cause corrosion of the metal reinforcement leading to degradation of this structure and spalling of the concrete material covering the reinforcing members. In this method an electrolyte is carried in a porous material between the outside surface of the concrete and the anode.
Examples of this method are shown and described in detail in a brochure by Norcure Chloride Removal Systems Inc. entitled "Is Salt Induced Corrosion Causing Problems with your Concrete Structures", in a brochure by Vector Construction entitled "The Concrete Restoration and Protection Specialists" and in a brochure by "Fosroc/NCTAS" entitled "Norcure Desalination". These brochures describe a technique which is used for various concrete structures including bridge decks and the brochure by Fosroc shows particularly a technique in which a bridge deck is restored using this anodic method.
In the brochure and as generally used in practice, after the concrete surface is exposed by removal of any covering layers, a porous material is laid down onto the upper surface and this receives an electrolyte. The porous material is then covered by a mesh type electrode in the form of wire netting which is then covered by a further layer of the porous material.
A current supply is connected between the mesh anode and the reinforcing steel of the concrete and over an extended period of many weeks this acts to cause the transfer of ions from the concrete material through the electrolyte to provide a restorative effect.
The increased usage of salt as a de-icing agent in freezing conditions has severely exacerbated the problem of chloride degradation of concrete structures.
Restoration of concrete using a temporary current is an entirely different process from impressed current cathodic protection. In the latter process, a small current typically of the order of 1-10 mAmps/sq meter is caused to flow continuously through the life of the concrete for the purpose of inhibiting corrosion.
The current used in the restoration process is strictly temporary for a period of the order of 20 to 90 days and has a value which of the order of 50 to 200 TIMES that of the continuous current. Thus the current in the restoration process may lie in the range 0.4 to 3.0 Amps/sq meter. In addition, the process of restoration must include a liquid electrolyte whereas the continuous process is typically dry. Therefore the types of anode and materials to be used are of an entirely different character.
In PCT Published Application WO94/29496 of Aston Material Services Limited is provided a method for cathodically protecting reinforcing members in concrete using a sacrificial anode such as zinc or zinc alloy. In this published application and in the commercially available product arising from the application, there is provided a puck-shaped anode body which has a coupling wire attached thereto. In the commercially available product there are in fact two such wires arranged diametrically opposed on the puck and extending outwardly therefrom as a flexible connection wire for attachment to an exposed steel reinforcement member.
The puck is surrounded by an encapsulating material such as mortar which holds an electrolyte that will sustain the activity of the anode. The mortar is compatible with the concrete so that electrolytic action can occur through the mortar into and through the concrete between the anode and the steel reinforcing member.
The main feature of the published application relates to the incorporation into the mortar of a component which will maintain the pH of the electrolyte in the area surrounding the anode at a high level of the order of 12 to 14.
In use of the device, a series of the anodes is provided with the anodes connected at spaced locations to the reinforcing members. The attachment by the coupling wire is a simple wrapping of the wire around the reinforcing bar. The anodes are placed in location adjacent to the reinforcing bars and re-covered with concrete to the required amount.
Generally this protection system is used for concrete structures which have been in place for some years sufficient for corrosion to start. In general, areas of damage where restoration is required are excavated to expose the reinforcing bars whereupon the protection devices in the form of the mortar covered puck are inserted into the concrete as described above and the concrete refilled.
These devices are beginning to achieve some commercial success and are presently being used in restoration processes. However improvements in operation and ergonomics are required to improve success of this product in the field.
In International Publication WO98/16670 of Bennett and Clear is disclosed another cathodic protection system intended to be used as a surface arrangement. This arrangement relates to a thinly sprayed zinc or zinc alloy which is applied onto the surface of the concrete. This zinc or zinc coating is then used as an anode to supply current for the cathodic protection process. As the anode is exposed at the surface, this may be used either as a sacrificial system in which there is no applied current and the anode is gradually eroded as the electrolytic process proceeds or as an impressed current cathodic protection system.
The improvement of the above Bennett application relates to the application of a humectant in free-flowing form which is positioned at or near the interface between the zinc anode coating and the concrete surface. It has been found and is disclosed in this application that the provision of the humectant in free-flowing form acts to absorb moisture from the area above the surface. The humectant is defined in the application as being either deliquescent or hygroscopic where a deliquescent material is one which becomes moist or liquefied after exposure to humid air and a hygroscopic material is defined as one which is capable of absorbing moisture from the atmosphere. The humectant is delivered to or near the interface of the anode by application as a solution which is aqueous, colloidal or in an organic solvent such as alcohol. When the humectant in solution is applied to the surface of the anode, it is transported to or near the interface by capillary action. The application states that the humectant is applied to the exposed surface of the anode coating and therefore the anode coating must be sufficiently thin or otherwise arranged to be porous to allow the humectant to reach the interface.